Final answer:
A trisomic individual in normally diploid organisms is designated as 2n + 1, indicating an extra chromosome beyond the normal diploid number. Trisomy can lead to genetic disorders like Down syndrome, with the extra chromosome causing a genetic imbalance and developmental challenges due to excessive gene dosage.
Step-by-step explanation:
In organisms that are normally diploid, a trisomic individual is designated as 2n + 1. Diploid organisms have two sets of chromosomes; for humans, this includes 22 pairs of autosomes and one pair of sex chromosomes, amounting to a euploid chromosome number of 46. A trisomic individual has an extra chromosome, resulting in an odd number of chromosomes, such as the characteristic chromosome 21 trisomy that leads to Down syndrome. The presence of an additional chromosome causes a genetic imbalance by adding a third copy of gene sequences which disrupts the usual gene product balance. This can lead to various developmental and functional challenges due to the excessive gene dosage.
An individual with a monosomy (2n - 1) usually cannot survive due to the absence of essential genes, while individuals with certain autosomal trisomies can survive for different durations, such as trisomy 13, 15, 18, 21, or 22. However, being polyploid (having more than two complete sets of chromosomes) is generally lethal in animals, but common and often advantageous in plants.